"If you have the choice, STEP is the preferred one since it’s superior in terms of the conversion process to IGES. The STL format can only be used for fluid mechanics analysis since only certain meshing algorithms on SimScale support it." Source: SimScale documentation
I also looked at other public projects dealing with spheres and they all had the same "seam" that you have.
By the way: Which forces do you mean? The ones from the "Result control" ?
If you need help for the simulation setup, feel free to ask.
P.S: Concerning your question about the warning, @afischer posted a good answer to this a while ago:
"The hexahedral meshing is not based on so called rich geometry formats as for example *.step or *.iges, but on triangulated geometries like e.g. *.stl. In general you should not care about this at all as the triangulation is done automatically prior to the meshing. In some cases, if the geometry model is not "clean", the triangulation can not be done properly and there might be small cracks or duplicated triangles. The meshing is in general very robust in regards to those defects. That is why your mesh is still valid, but we expose this as a warning as it might help in cases where the mesh result does not look as expected."